More Payload Isn't Always Better:
Managing Hydrophobicity in High-DAR ADCs with pSar

Increasing drug-to-antibody ratio can increase the amount of payload carried by an antibody-drug conjugate. But higher drug loading can also increase ADC hydrophobicity, potentially affecting aggregation, clearance, pharmacokinetics and in-vivo performance. Hydrophilic linker engineering provides one strategy for addressing this challenge.

The High-DAR Challenge

Drug-to-antibody ratio describes the average number of drug molecules conjugated to each antibody.

Increasing DAR may increase payload delivery, but many potent ADC payloads are hydrophobic.

As additional payload molecules are attached, the physicochemical properties of the entire conjugate can change.

Increased hydrophobicity may contribute to:

✓ Aggregation

✓ Reduced aqueous solubility

✓ Non-specific interactions

✓ Faster systemic clearance

✓ Altered pharmacokinetic performance

Can a Hydrophilic Shield Help?

One strategy is to incorporate a hydrophilic component into the drug-linker architecture.

The goal is to partially mask the hydrophobic character of the payload while retaining the desired drug loading.

PEG has long been used as a hydrophilic spacer in bioconjugation. Another material being explored for this purpose is polysarcosine (pSar).

What Is Polysarcosine?

Polysarcosine is a hydrophilic polypeptoid composed of repeating N-methyl glycine units.

Structurally defined, monodisperse pSar molecules can be used as hydrophilic spacers or masking groups in bioconjugation and drug-delivery systems.

Because discrete chain lengths can be synthesized, researchers can systematically evaluate how polymer length and linker architecture influence conjugate behavior.

A Homogeneous DAR 8 ADC

A 2019 Chemical Science study by Viricel and colleagues used monodisperse polysarcosine as a hydrophobicity-masking element in highly loaded ADCs.

The researchers generated homogeneous DAR 8 conjugates using a hydrophilic pSar-containing drug-linker platform.

The strategy improved physicochemical properties while supporting high drug loading, and the study reported improved pharmacokinetics and in-vivo antitumor efficacy compared with an unmasked comparator.

This suggests an alternative ADC-design question: instead of simply reducing DAR, can linker chemistry make high-DAR conjugates more manageable?

Linker Topology Matters

The study also demonstrated that simply adding a hydrophilic polymer was not sufficient to define performance.

The relative orientation of pSar and the payload influenced hydrophobic masking, with a more orthogonal arrangement providing better shielding than a more linear architecture in the tested constructs.

In other words, where the hydrophilic group is positioned can matter as much as which hydrophilic group is used.

Longer Is Not Necessarily Better

The relationship between pSar chain length and conjugate performance was not simply linear.

An optimal chain-length window was observed rather than a “the longer, the better” relationship.

ADC optimization may therefore require evaluation of payload hydrophobicity × target DAR × polymer length × linker topology as an integrated design problem.

PEG or pSar?

pSar should not simply be viewed as a universal replacement for PEG. PEG remains widely used across bioconjugation and drug-delivery applications.

Instead, polysarcosine provides an additional hydrophilic design option that can be evaluated when physicochemical properties become limiting.

The appropriate choice depends on the payload, conjugation chemistry, linker architecture and performance requirements of the final conjugate.

Designing a pSar Linker

Chain Length
Discrete pSar lengths allow systematic evaluation of hydrophilic shielding.

Terminal Functionality
Carboxyl, amine, azide, maleimide and other functional groups can support different conjugation strategies.

Topology
The relative position of the polymer and payload may influence shielding efficiency.

Payload Properties
The hydrophobicity and chemistry of the payload should be considered.

Target DAR
Drug loading should be optimized together with the physicochemical behavior of the complete ADC.

Structurally Defined pSar Building Blocks

Precise PEG currently provides a broad collection of structurally defined pSar linkers with multiple chain lengths and terminal functionalities.

Examples include Ac-pSar, Boc-protected pSar, azide-functionalized structures and pSar-containing maleimide/linker architectures.

Custom linker synthesis can also provide additional options for ADC, protein-conjugation and drug-delivery research.

Developing an ADC Linker or Bioconjugation Strategy?

Dana Bioscience can help identify ADC linkers, PEG/pSar spacers, click-chemistry reagents and bioconjugation building blocks.

Share your payload, target DAR, desired chain length, conjugation chemistry and terminal functionality, and we can help evaluate available research products and custom linker options.